Yes, the gearing needs to be addressed. What year Express do you have? The 82-83 models seem to have more space between the transmission and the tire sidewall than the 77-80 models. The 81 may be similar or should be to the 82-83 models but I'd have to check.
I've got a IRC 70/90 x 14" tire on the rear with plenty of room to spare. So you could go to even an 80/90 x 14" tire - of course, no more rear fender, at least, attached the traditional way.
80mm tire will be almost 3/4" of an inch TALLER (overall diameter) than the 70/90 and over 1.5" taller than the stock tire. So that's 3.6 mph faster than stock tire and 1.5 mph faster than 70/90 tire assuming engine can turn bigger tire at same rate as smaller tire.
What would be interesting would be to lace up a 16" x 2" rear and see if I can squeeze it in there. But it wouldn't get me much over an 80/90 x 14 except maybe weight savings.
I've been at 42 on the little Express and it screams so much, you can't help but think that this is not a good thing